The present invention relates to anaesthetic vaporisers and in particular to anaesthetic vaporisers of the by-pass type. UK Patent No. 1224478, describes an anaesthetic vaporiser of the by-pass type in which a carrier gas such as oxygen, air or nitrous oxide is initially divided on entry to the vaporiser between a first stream which is directed towards the sump or vaporising chamber of the vaporiser to entrain vapour from a volatile liquid anaesthetic contained therein; and a second by-pass stream, the first and second streams subsequently re-combining prior to leaving the vaporiser for delivery to a patient.
In UK Patent No. 1224478, there is described a thermally controlled valve which is located in the second, by-pass stream. The function of the thermally controlled valve is to adjust the quantity of carrier gas in the by-pass stream. As the temperature of the vaporiser increases the resistance of the thermally controlled valve to the flow of the carrier gas through the by-pass stream decreases thereby allowing more carrier gas to pass through it, and on recombining with the first stream emerging from the vaporiser sump produces a gas mixture containing the same proportion of anaesthetic drug as it does at the initial temperature. By locating the thermally responsive valve in the second by-pass stream the following technical advantages accrue:
(a) Some volatile liquid anaesthetic agents are corrosive in the presence of water vapour which is know to be present in some medical gas supplies and placing the thermally responsive valve in the by-pass stream protects this very sensitive device from the effects of such corrosion.
(b) The sensitivity of the thermally responsive valve may be judged by the fact that a concentration change by a factor of 0.05 occurs with a valve movement of 0.0001". Even this movement is however, coarse by comparison with the smaller movement which would have to be controlled if the thermally responsive valve was located in the first stream of the vaporiser where the flow rate of gas is much lower.
(c) The anaesthetic agent HALOTHANE contains thymol as a stabiliser and this can be left by evaporation on operating surfaces. Siting the thermally responsive valve in the second by-pass stream means that this most sensitive component is protected from the effects of thymol build-up.
It is believed, that the beneficial technical effects of placing the thermally responsive valve in the second by-pass stream has resulted in the popularity of anaesthetic vaporisers embodying the features described in UK Patent 1224478 with the medical profession.
However, the thermally controlled valve described in UK Patent No. 1224478, is located at a position spaced from the vaporising chamber approximately in the middle of the vaporiser which gives rise to the following disadvantages:
(a) There is a time delay in the thermal response of the valve following any temperature change of the liquid anaesthetic agent in the sump of the vaporiser; and
(b) The thermally responsive valve is relatively inaccessible for adjustment and maintenance purposes.